OSC has been around a long time, and has morphed into what can only be called a big pot of spaghetti. The code is messy, hard to follow, upgrades are awful, and the default install doesn't have nearly the functionality of zencart. I haven't used OSC in a long time, and I know they've switched to the templating system.. so it might be a little better now. ZenCart has always been on the templating system (which allows you to upgrade without overwriting most of your mods) and they're installation process is an absolute breeze. Documentation is wonderful, the code is somewhat straightforward (note: somewhat, not completely), and the community is just as helpful (if not more so) than OSC. Plus mods like gift certificates and coupon discounts were already in place.
